In my aforesaid co-pending applications there are described apparatus and methods for treating used asphalt-aggregate compositions. Particularly, these compositions are described as being crushed and separated into the various particle size ranges. The coarse particles are introduced into the heating apparatus in a hot zone, normally adjacent the drum end into which flame and hot gases of combustion are supplied, opposite the cooler output apparatus end. Smaller particles are separately introduced downstream from the hot drum end toward the cooler drum end, in a cooler zone whereby burning of the more readily heated asphalt containing particles is avoided. The compositions are normally separated into two or more particle size ranges. The description of the particle size ranges used in the process of the invention is the same as disclosed in my aforesaid co-pending applications, and is incorporated herein by reference.
Although it is understood that the coarse particles themselves will not become as readily heated as smaller size particles because of the difference in the ratio of the surface area and the mass of the particle, when introducing coarse particles in the hottest area of the drum, it has been found that some fine particles are also present, due to incomplete separation of the particles. The relatively fine particles present in the coarse particle size range may also result from break up of coarse particles during transfer from the stockpile to the drum, as they are handled by scoops, conveyors, and the like. Thus, even with efficient separation of crushed asphalt-aggregate particles into various size ranges, because of the necessary handling and transfer of these compositions between storage areas and from stockpiles to the heating and mixing apparatus, some particles sizes outside any given range may be formed, especially fine particles from the coarse and intermediate particles. The presence of fine particles presents a more serious problem when introducing coarse particles into the hottest apparatus end where they are exposed directly to the hot gases of combustion and flame supplied. Although the coarse particles which are alternately lifted and dropped in a cascade fashion through these hot gases are not burned but become more slowly heated, as described in my aforesaid co-pending applications, fine particles so treated may become rapidly overheated in the 1,000.degree.-2,000.degree. F or more temperatures, causing burning of the asphalt and concomitant smoke and asphalt degradation. It is to the elimination of these problems that the present invention is directed.